The electropyrotechnic initiators integrated in gas generators traditionally comprise an ignition support, and electrical initiation system connecting the ignition support to a source of electricity, and a fragmentable cap containing at least one electropyrotechnic composition.
This case-forming cap, usually made of metal, is held inside the generator by mechanical fastening and connection means. It is sealed and isolated from the outside.
In operation, the pressure rises inside the cap and the cap opens in its weakest zone, thereby releasing a flame and hot gases which, in turn, initiate an electropyrotechnic material present in the generator, such as a propellant, suitable for generating gas.
The zone of weakness is generally created by rupture-starters, which are obtained by stamping indented patterns, in particular when the case is made by stamping.
Said cap is generally cylindrical in shape, presenting an open first end connected to the ignition support, an opposite second end closed by an end wall, and a cylindrical side wall.
Until now, many initiators have presented rupture-starters in the end wall, thereby allowing the flame to exit axially.
Nevertheless, certain generators, in particular those having a mass of propellant in the form of a ring surrounding the initiator, or in which the electropyrotechnic charge is so small that rupturing the axial end wall would run the risk of damaging the charge, operate under better conditions when the flame exits radially, i.e. through the side wall of the cap, via holes that are closed while not in operation by an inside wall of plastics material.
Other structures allow the flame to exit radially.
Thus, U.S. Pat. No. 5,005,486 (Lenzen) describes a gas generator which includes an initiator whose side wall has zones of weakness disposed parallel to the generator lines of the cylinder in which the cap of the initiator is defined. Those zones extend from one end of the cap to the other.
In practice, when the zones of weakness give way, the initiator opens up completely to such an extent that the integrity of its structure is affected.
Another structure described in U.S. Pat. No. 5,601,308 (Cuevas).
In that case, the zone of weakness forms a circumferential line going round the side wall of the cap of the initiator.
In that case also, the integrity of the initiator is affected, since after the zone of weakness has ruptured, one complete portion of the initiator becomes detached from the remainder.
Complete rupture of the initiator can allow particles to escape, in particular metal particles, that run the risk of being expelled to the outside.
An object of the present invention is to solve those problems by proposing specifically an initiator in which the (cylindrical) peripheral side wall of the cap presents a zone of weakness, but conserves its integrity, even after said zone has ruptured.
Another aim of the invention is to achieve the above-specified object without significantly changing the usual “architecture” of the initiator.
The invention thus provides an electropyrotechnic initiator which comprises:                an ignition support;        an electrical initiation system connecting the ignition support to a source of electricity; and        a cap containing at least one explosive composition, the cap being of generally cylindrical shape presenting an open first end connected to the ignition support, an opposite second end that is closed by an end wall, and a peripheral side wall which presents at least one zone of weakness.        
According to the invention, at least one of the zones of weakness extends along a non-closed trace and over a portion only of the side wall, i.e. without reaching its opposite ends.
The term “non-closed trace” is used to mean that the trace presents at least two distinct ends that do not coincide.
By means of the particular configuration of said trace, there is no rupturing of the initiator into a plurality of distinct and separate pieces. It conserves its integrity without affecting the immediate surroundings.
In another embodiment of the invention, the non-closed trace is shaped in such a manner that after each zone of weakness has opened, a fragment of the wall situated between two adjacent ends of the trace remains attached to the wall.
This wall fragment or “petal” remains attached to the wall of the initiator, without any risk of damaging the surrounding component, and it can act as a deflector for channeling the flame and the gas.
According to other, non-limiting characteristics:                the trace is cruciform;        the trace is in the form of a Saint Andrew's cross;        the trace is of curved shape;        the trace is in the form of a circular arc;        the trace is horseshoe-shaped;        that each zone of weakness is constituted by a thinning of material; and        each zone of weakness is obtained by stamping.        
Other characteristics and advantages of the present invention appear on reading the following detailed description made with reference to the accompanying drawings, in which: